I think it's peer pressure. We live in quite a prosperous area with lots of sports mad skinny types. Therefore, you think it's normal to be a healthy size. Perhaps if you lived somewhere where people are big, you accept that putting on weight is normal.
To be honest, I've always been in the correct BMI and I put it down to
3 proper meals a day at normal times. However, I don't have a nasty commute, work averagely hard, and have time to be normal. Plus, sent children to nearest school to give us all an opportunity for a walk each day. (E.g. making our routine involve some exercise).
Modern life conspires to make you fat - whereas in the past working meant toiling on land, burning up energy, now it means sitting in an office or car. I was my thinnest when on maternity leave with two children - constantly carrying, running up and down stairs etc.
At primary level, my children do tons of sport and there are loads of cooking projects. (They think they do too much sport - Friday involved rugby for an hour, a cross-country run and walking 3miles to the rugby and back!)
As for children in buggies - well a four year old who has been at school all day and has a 1.5 mile walk home probably needs a buggy ride. Other four year olds would be picked up by car. I'm very carefully on the buggy judging thing - you have no idea how much activity the child might have done before finally collasping in buggy!